In a shipyard in Scotland the future of the Royal Navy is slowly taking shape. But the construction of the aircraft carrier HMS Queen Elizabeth is a mammoth task.

Imagine an aircraft carrier as a 65,000-tonne jigsaw puzzle and you have got a good idea of the scale of the building of HMS Queen Elizabeth.

The pieces are being built at six shipyards around the UK and will be slotted together at Rosyth in Fife using an enormous crane which was transported by sea from China.

Around 10,000 workers across Britain are employed on the Â£5bn project with up to 25,000 engaged in building components for the Lockheed Martin F-35 aircraft.

The carrier will have between 12 and 40 F-35s, or Joint Strike Fighters, costing around Â£65m each.

The world's carriers

US: 11, with one under construction

Russia: One, the Admiral Kuznetsov

UK: One, HMS Illustrious which only carries helicopters - two under construction

China: None but is thought to be building several

France: One, the Charles de Gaulle

India: One, the Viraat, formerly known as HMS Hermes, but converting another, the Admiral Gorshkov, into the Vikramaditya. A third is under construction

Italy: Two, the Giuseppe Garibaldi and the Cavour

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"It's the biggest shipbuilding project for the Royal Navy ever and is second only in engineering terms to the Olympics," says the man in charge of the whole project, David Downs, engineering director with the Aircraft Carrier Alliance (ACA) consortium.

"All my nights are sleepless, worrying about it," he jokes.

The Queen Elizabeth and sister ship Prince of Wales will be far bigger than the Ark Royal but still significantly smaller than US equivalents.

Downs and his team designed the ships using computer software - every inch mapped out electronically with laser-guided measurements which ensure each part fits together.

Uniquely, a team of assessors from Lloyd's Register are on hand at all the yards to check the work as it proceeds.

BAE Systems is part of ACA and at its Govan yard in Glasgow, integrated work teams manager David Thomas gives a tour around one huge segment of the ship.

Things have changed - in 1924 the Navy's aircraft carriers were converted ships

Clambering under the hulk, wearing only a hard hat for protection, it's hard not to think of what would happen if the frame holding up 14,000 tons of steel gave way. But Thomas is reassuring on the yard's safety record.

He has been supervising the insertion of some of the 450 prefabricated cabins and 150 shower rooms - made by a firm on Teesside - in the ship's innards. He carries with him a small manual showing where everything fitted.

Anyone who finds the instructions to flatpack furniture a challenge would find it mind-boggling.

The whole process starts with the arrival of huge sheets of steel. They are "burned" into various shapes and sizes - some of them quite small - which are welded into position.

The scale of it

65,000 tonnes at full displacement

1.5 million square metres of paintwork

33 tonnes - weight of propellers

25 knots top speed

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Gradually the sections become bigger as deck after deck is welded together.

One of the Govan team is Lyn Gordon, 23, an apprentice fabricator and one of a number of women working on the project.

"My fascination with shipbuilding came from living on the Clyde," she says. "I realise that it will eventually be an aircraft carrier and I will get to see it turning from a sheet of a metal, to a component, to HMS Queen Elizabeth."

The first segment from Govan should be ready this summer and will be towed by barge, around the northern tip of Scotland, to Rosyth.

This piece, less than 1% of the entire ship, was brought to Scotland from Devon by barge

At Rosyth the dry dock is ready for the assembly process. Last month the crane arrived from Shanghai, having squeezed under the Forth Bridge at low tide.

Rosyth has List X status, meaning everyone working there has to be security cleared, including the 50 Chinese workers who are helping to erect the 93m crane.

The first piece of steel was cut in 2009 but HMS Queen Elizabeth will not be finished until 2016 at the earliest, and may not be ready for action until 2020.

The construction of her sister ship, the HMS Prince of Wales, will overlap and the current plan is for one of them to be operational while the other would be kept in "extended readiness".

With the Ark Royal's fleet of Harrier jump jets being decommissioned the Navy will be without carrier-based planes for almost a decade.

Recent events in Libya have showed the importance of mobile air power.

The dry dock at Rosyth where the ship will be assembled, with the giant crane in the background

The MoD complicated matters in October when it decided, in the Strategic Defence Review, to fit the carriers with catapults and arrester wires.

The "cats and traps" will enable them to fly the carrier variant F-35 and will also enable US and French jets to land on the deck. But it will also delay the completion of the carriers.

"If they get the two ships in the form they are expected they will be enormously capable ships. It's like having a piece of Britain you can place anywhere in the world," says naval historian Nick Hewitt.

Aircraft carriers are arguably the ultimate symbol of military prestige, a mobile projection of military might.

Lyn Gordon is proud of Clydeside's shipbuilding heritage

The Royal Navy pioneered carriers, explains Hewitt, head of attractions and collections at the Portsmouth Naval Base Property Trust. The first carriers were converted ships like the 7,500 ton Ark Royal, whose biplanes first saw action in February 1915 against the Turks in the Dardanelles.

Since the 1930s, US carriers have dwarfed their British allies, Hewitt notes.

"The US carriers were designed for the Pacific and to be away from base indefinitely. The British carriers were designed to operate in the North Sea, the Mediterranean and the North Atlantic or from bases in Singapore or India."

Shipbuilding jargon

Black trades - welders and burners

Caulkers - people who grind and buff the welded metal to make it watertight

Bulkhead - a steel wall within the ship, which can be sealed. Designed to prevent sinking if ship is hit

Sponson - a structure which juts out from the side of the carrier deck

Wet space - shower room, usually prefabricated and hoisted into place

Lockout - components which are included as part of the building programme

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HMS Invincible, which fought in the Falklands, and HMS Ark Royal, which was recently pensioned off, weighed in at a puny 22,000 tons compared to the American carriers such as the USS George H W Bush, at 101,000 tons. The QE Class weighs in in between - at 65,000 tonnes full displacement.

When it is finally ready the Queen Elizabeth will only be able to navigate the Forth Bridge and reach the open sea by waiting for low tide, and even then they will have to retract the radar masts.

The project has had its critics.

The former deputy chairman of Babcock - which is part of the ACA - Lord Hesketh resigned in November after describing the project as a "disaster".

He told the BBC the carriers could have been built for a fraction of the cost at a shipyard in South Korea and claims the project only went ahead in its present form because of the number of jobs it preserved.

But whatever the controversy over the carriers and the cost, the effort involved will be phenomenal.